gesture-action: Prepare for clutter_gesture_action_cancel() within ::gesture-end

There may be odd situations where full gesture cancellation may be wanted at once
when the first touch is lifted and ::gesture-end is emitted on a gesture action.
Although calling clutter_gesture_action_cancel() within the ::gesture-end handler
causes 2 critical warnings that are otherwise harmless.

https://bugzilla.gnome.org/show_bug.cgi?id=732907